In a communication system, carrier aggregation is applied to support wider bandwidth and accomplish the peak value rate. In the carrier aggregation technology, multiple branch carriers are aggregated to support wideband transmission. Every branch carrier is backward-compatible. Depending on capabilities of User Equipment (UE), the UE can receive or send signals of multiple carriers simultaneously.
The communication between the UE and the Base Station (BS) is generally based on a Hybrid Automatic Repeat Request (HARD) technology in the process of sending/receiving data of each branch carrier. That is, the data of a transmission block is code-modulated and sent; after the receiver receives the data, if the data passes the Cyclical Redundancy Check (CRC), the receiver regards the decoding as correct and returns an ACKnowledgement (ACK) message; if the data fails the CRC, the receiver regards the decoding as incorrect, and returns a Non-ACKnowledgement (NACK) message. The ACK message and the NACK message are collectively known as confirmation messages, and the transmitter performs further operations such as HARQ according to the received confirmation message.
In the sending or receiving of the data of multiple carriers, an independent HARQ process of each carrier is applied. For example, data may be sent over a physical shared channel, and control information may be sent over a physical control channel. Physical Downlink Control Channel (PDCCH) is made up of Control Channel Elements (CCEs). A physical control channel is generally made up of 1, 2, 4, or 8 CCEs. When multiple carriers exist, each carrier has an independent HARQ process so that confirmation information may need to be sent by multiple Physical Uplink Control Channels (PUCCHs).
In the case of a single carrier, the resources for the uplink carrier to feed back the ACK/NACK are reserved according to the maximum number of CCEs on the corresponding downlink carrier.
In the case that the carrier aggregation scheme is aggregation of paired carriers, namely, the number of uplink carriers is equal to the number of downlink carriers, the single-carrier rule is still applicable to reserving and mapping of the ACK/NACK channel resources. However, in the case that the carrier aggregation scheme is aggregation of paired carriers, especially aggregation of UE-specific non-paired carriers, the single-carrier rule is no longer applicable to reserving and mapping of the ACK/NACK channel resources. Because the corresponding feedback channel is uncertain, it is impossible to feed back the ACK/NACK messages corresponding to non-paired downlink carriers or uplink carriers.